Background: The management of suicidal crisis remains a major issue for clinicians, driving the development of new strategies to improve suicide prevention. Methods: We conducted a randomized controlled trial comparing a 7-week acceptance and commitment therapy (ACT) versus relaxation group, as adjunct to treatment as usual for adult outpatients suffering from a current suicidal behavior disorder. The primary outcome was the rate of change in the Columbia Suicide Severity Rating Scale suicidal ideation subscore (adding severity and intensity subscores). Secondary outcomes were the rates of change for depressive symptomatology, psychological pain, anxiety, hopelessness, anger, quality of life, and therapeutic processes. Assessments were performed in the 2 weeks preceding the beginning of the treatment (pretreatment assessment), and within 1 week (posttherapy assessment) and 3 months (follow-up assessment) after therapy completion. Results: Forty adults were included and randomized. The rate of change in ACT for suicidal ideation at the posttherapy assessment was higher than in the relaxation group (β [SE] = –1.88 [0.34] vs. –0.79 [0.37], respectively; p = 0.03). ACT effectiveness remained stable at the 3-month follow-up. We found a similar pattern of change for depressive symptomatology and anxiety, psychological pain, hopelessness, anger, and quality of life. Therapeutic processes improved more in the ACT group than in the relaxation group. Treatment adherence was high in the ACT group, all participants reported satisfaction with the program. Conclusions: Through its effectiveness in reducing suicidal ideation and improving the clinical dimensions associated with suicidal risk in patients suffering from a suicidal behavior disorder, ACT could be developed as an adjunctive strategy in programs for suicide prevention.